The present invention relates to oscillators. More specifically, it relates to improvements in crystal oscillators in which high-frequency stability must be achieved while consuming a minimum amount of power.
In many applications, the frequency of a quartz crystal oscillator must remain stable despite changing ambient temperatures. For example, in a positioning system using beacons on a target, and using satellites to track the target, the positioning function is implemented using an analysis of the Doppler shift caused by the relative motion of the satellite and target. A crystal oscillator in the target transmitter must have a stable frequency output, because any change in the frequency could cause the positioning system to interpret the frequency change as a Doppler shift, resulting in a positioning error. The stabilization of a crystal oscillator's frequency in the presence of changing temperatures, with the added demand that minimum power be consumed in a battery-powered unit, presents a challenge to the designer of the crystal oscillator.
One known solution is to temperature-compensate the oscillator. A voltage variable capacitor is added to the oscillator so that the frequency can be shifted a small amount by a correction voltage developed by a thermistor network. This correction voltage causes the oscillator frequency to remain substantially constant as the ambient temperature changes. Because perfect cancellation is not possible, there remains some residual frequency drift as a function of temperature. Additionally, the frequency correction network degrades the oscillator's phase noise characteristics and short term stability.
Another known solution is to maintain the entire oscillator including the quartz resonator at a precisely controlled elevated temperature, higher than the greatest expected ambient temperature. The oscillator temperature is regulated by a thermostat or a proportional controller. Excellent frequency stability is possible if a sufficiently accurate temperature feedback loop is employed. A temperature-regulated oscillator has far better frequency stability than the temperature-compensated oscillator described in the previous paragraph, and suffers no phase noise degradation However, for many battery-powered applications, the heater's electrical power requirement is impractically high.
It is therefore desirable to provide a low-power temperature-controlled quartz crystal oscillator intended for battery-powered applications.
Within the confines of standard practice, electrical heating power may be reduced in two ways. First, the thermal insulation surrounding the oscillator can be increased or improved. Second, the "set temperature" (the temperature to which the oscillator components are heated and maintained) can be lowered.
Unfortunately, only minor improvements are possible with either of these known approaches.
First insulation volume cannot be increased without increasing the overall size of the oscillator. This increase in size is generally not desirable or possible. Also, common insulation materials do not lend themselves to further development.
Second, lowering the set temperature introduces difficulties originating in the frequency-temperature characteristics of the quartz crystal resonator.
FIGS. 7A and 7B show two representative examples of known crystal resonators. In both cases the large circular body 114 is the quartz crystal resonator. Known crystal resonators generally consist of a slice of crystalline quartz, usually circular, with metallic electrodes deposited on each side. The illustrated resonator 114 has one circular electrode 115 deposited on each side of the resonator. Each electrode has a rectangular extension protruding to the edge of the resonator.
Two posts 108 provide mechanical support and electrical connection to the electrodes. These posts are electrically isolated from the resonator enclosure by glass seals 109.
The frequency-temperature characteristics of quartz crystal resonators are determined by the orientation of the quartz slice with respect to the natural axis of the crystal from which it is cut. The frequency-temperature coefficient can be made zero at a specific temperature by properly orienting the quartz slice. However, the temperature coefficient increases rapidly as the resonator temperature moves away from this optimum temperature. Temperature-controlled oscillators take advantage of the zero coefficient point to minimize the effects of ambient temperature variations. This means that the set point of the oscillator and the zero coefficient point of the resonator must be made to coincide. Accordingly, a group of quartz crystal resonators (specifically the AT-, IT- and SC-cut resonators) have been developed with a range of zero temperature coefficients occurring above the normally expected range of ambient temperatures, typically 75.degree. C. to 85.degree. C.
Although the zero-coefficient temperature can be varied by slight changes in crystal orientation, the extent of this variation is limited. This limited variability of zero-coefficient temperature limits the set temperature of conventional temperature controlled oscillators.
Temperature regulation of a conventional temperature-controlled oscillator is lost if the ambient temperature rises above the set temperature. The heater shuts down, and the oscillator follows the resonator's frequency-temperature curve. With traditional AT-, IT- or SC-cut resonators, frequency accuracy degrades rapidly. Loss of temperature regulation and limited control of the resonator zero-coefficient temperature place a practical lower limit on the set temperature.
As described in greater detail below, an alternative crystal cut, the FC-cut, is used by the present invention to overcome these limitations. First, the inventive crystal oscillator takes advantage of the realization that the zero temperature coefficient point can be set much lower than that of traditional AT-, IT- or SC-cut resonators. Second, with proper orientation, the zero-coefficient range can be made very broad. Specifically, an FC-cut crystal resonator of proper orientation possesses the frequency-temperature relationship shown in FIG. 6. At low temperatures, the frequency-temperature coefficient is unacceptably large and positive (shown in segment 602). Segment 602 is followed by a broad segment 604 in which the frequency-temperature curve is substantially flat. At high temperatures (illustrated as segment 606), the frequency-temperature coefficient again becomes unacceptably large.
With an FC-cut resonator such as that used in embodiments of the present invention, the oven set temperature can be placed on the lowest point in the substantially flat region 604. As the ambient temperature rises above the set temperature, the oscillator heater shuts down. Temperature regulation is lost, but unlike conventional temperature-controlled oscillators using AT-, IT- or SC-cut resonators, frequency stability remains acceptable. Therefore, the power savings of a low set point is realized without degrading frequency accuracy at high ambient temperatures.
It is also possible to reduce heat loss (and thereby reduce energy consumption) by reducing the volume of the heated enclosure. In known practice, both the crystal resonator and the oscillator circuitry are contained within a common temperature-controlled enclosure. The oscillator is elevated to nearly the same temperature as the resonator by means of the heated air contained within the enclosure. The large volume conventionally needed to enclose oscillator circuitry increases the quantity of electrical power required to maintain the elevated resonator temperature.
Because the crystal resonator has a very high Q, it is the crystal which substantially determines the frequency of oscillation. Remaining oscillator components have relatively little effect on the frequency of oscillation, even for temperature-induced parameter changes. Embodiments of the present invention take advantage of the realizations that heating power can be reduced by maintaining only the resonator at a constant elevated temperature, and that small frequency shifts caused by temperature-induced changes in other oscillator components can be safely disregarded.
The invention also takes advantage of the realization that additional power reduction can be achieved by applying heat directly to the resonator. In conventional practice, the resonator enclosure, not the resonator itself, is heated. Because the resonator enclosure is evacuated, heat is not conducted efficiently into the resonator. The only significant path for heat conduction into the resonator is through the resonator support posts 108. These posts, being long and thin, present a significant thermal resistance. ("Thermal resistance", in this sense, refers to a resistance to heat flow.)
The posts themselves are insulated from the resonator enclosure by glass seals 109. The glass seals 109 not only provide electrical insulation for the resonator support posts 108 where they penetrate the enclosure 104, but also serve to thermally insulate the support pins, thereby constituting a portion of the significant thermal resistance between the resonator 114 and its enclosure 104.
In the present invention, the previously unexploited thermal isolation of the resonator is used to advantage. According to embodiments of the present invention, the resonator is heated by an electrically resistive element deposited directly on its surface. In this implementation, the thermal resistance of the resonator support pins helps reduce the overall oscillator power consumption. Other oscillator components are warmed only to a small degree by the heat that incidentally leaks out of the enclosed resonator.
Efficient control of heating current is necessary to realize a low-power temperature-controlled oscillator.
In conventional practice, a variable-resistance pass element, typically a transistor, is used to vary the heating current. The principle of voltage division ensures that, undesirably, a sizable portion of heater power is dissipated in the control device instead of the heating element. Usually, this heat is recovered by mounting the control element directly on the temperature-controlled structure.
In contrast, according to the present invention, heat dissipation in the heater control device is effectively eliminated by employing pulse width modulation of the heater current. Heating power is conserved by applying it directly and solely to the resonator.
In summary, the present invention overcomes the disadvantages of known temperature controlled oscillators, as described above. As will be appreciated by reading the following disclosure, heating power expenditure is minimized by one or more of the following features
(1) employing a lowered set temperature made feasible by use of an unconventional FC-cut crystal resonator; PA1 (2) a reduction of the heated volume, thereby reducing heat loss; PA1 (3) use of previously unexploited thermal isolation of the resonator by heating the resonator directly with a heating element (such as a resistive heating element), the heating element placed substantially directly on the surface of the resonator; and/or PA1 (4) use of pulse width modulation to control the heating current with maximum efficiency.
Furthermore, various embodiments of the present invention may include one or more of the following features, as recited in the claims which follow this specification.
A resonator, which may be a quartz crystal, is situated in a substantially evacuated enclosure so as to reduce thermal conductivity between the resonator and its enclosure. A heating element is deposited substantially directly on the resonator surface. The heating element is designed to controllably add thermal energy to maintain the resonator at a desired temperature.
A temperature sensor is attached to the crystal enclosure, and may be sandwiched between the crystal enclosure and the circuit board to which the crystal enclosure is attached. A control unit converts the sensed temperature into signal (such as a series of variable-width pulses) to be applied to the resonator heating element. Thus, the sensor, control unit and heating element comprise a temperature-feedback control system which allows the crystal resonator to reliably operate at or very near its desired temperature.
The crystal enclosure and accompanying oscillator circuitry may be surrounded by thermally insulative material, thereby further reducing the power consumption of the unit. Thermal insulation minimizes heat loss from the resonator, and also plays a substantial role in the function of the inventive temperature-feedback control system.
Furthermore, using an FC-cut resonator in certain preferred embodiments allows the resonator set temperature to be well below the highest expected ambient temperature without loss of frequency accuracy, thus achieving additional power savings. The minimum temperature coefficient region of an FC-cut resonator falls at a temperature much lower then that of crystal cuts commonly used in temperature controlled oscillators. The minimum temperature coefficient region is also much broader. The set temperature can therefore be lowered without incurring an unacceptable frequency deviation at high ambient temperatures.
In an especially preferred embodiment, an integrated circuit originally intended for use in switching power supplies may be used in a novel manner to perform certain of the functions in the temperature feedback loop control circuitry, thus simplifying implementation of these functions.
Other objects, features, and advantages of the present invention will become apparent upon a reading of the accompanying disclosure.